Insulin resistance contributes to the pathogenesis of several human diseases such as obesity and non-insulin-dependent diabetes mellitus. We have investigated patients with genetic forms of extreme insulin resistance to gain insight into biochemical defects which give rise to disease. Thus far, we have identified and characterized three mutant alleles of the insulin receptor gene. One insulin resistant patient (leprechaun/Ark-1) is a compound heterozygote who has inherited two alleles with distinct mutations. One allele has a missense mutation causing the substitution of glutamic acid for lysine at position 460 in the alpha-subunit of the receptor. This mutation increases the affinity of the receptor to bind insulin, and decreases the ability of acid pH to dissociate insulin from its receptor. The second allele has a nonsense mutation in which codon-672 is converted to a chain termination codon. This truncated receptor lacks the C-terminal, 48 amino acids of the alpha-subunit and the entire beta-subunit. Furthermore, the truncated receptor appears to be degraded rapidly and is not expressed at the cell surface. We have also identified two insulin resistant sisters from a consanguineous family. Both sisters are homozygous for a alpha-subunit. This mutation is associated with impaired transport of the receptor to the plasma membrane. An additional mechanism of insulin resistance is associated with a decrease in the levels of insulin receptor mRNA causing a decrease in the number of insulin receptor. We are currently cloning and characterizing the regulatory regions of the insulin receptor genes from these patients.